Confirmation of mission success came 13 hours after the actual flyby and, after a day of both jubilation and tension, allowed the New Horizons team to finally celebrate in full force.

Early indications had been encouraging, and a cheering, flag-waving celebration swept over the mission operations center in Maryland at the time of closest approach yesterday morning.

But until New Horizons phoned home last night, there was no guarantee the spacecraft had buzzed the small, icy, faraway, but no longer unknown world.

The unprecedented encounter was the last stop on NASA's grand tour of the planets over the past half-century. New Horizons' journey began 9½ years ago, back when Pluto was still considered a full-fledged planet.

"This is truly a hallmark in human history," said John Grunsfeld, NASA's science mission chief. "It's been an incredible voyage." According to NASA, the spacecraft the size of a baby grand piano swept to within 7,700 miles of Pluto at 31,000 mph. It was programmed to then go past the dwarf planet and begin studying its far side.

To commemorate the moment of closest approach, scientists released the best picture yet of Pluto, taken on the eve of the flyby.